This invention relates to a blasting process, and more particularly, to a process for explosively rubblizing underground oil shale retorts, and blasting oil and gas wells.
Researchers have now renewed their efforts to find alternative sources of energy and hydrocarbons in view of recent rapid increases in the price of crude oil and natural gas. Much research has been focused on recovering hydrocarbons from solid hydrocarbon-containing material, such as oil shale, coal, and tar sands, by pyrolysis or upon gasification to convert the solid hydrocarbon-containing material into more readily usable gaseous and liquid hydrocarbons.
Vast natural deposits of oil shale found in the United States and elsewhere contain appreciable quantities of organic matter known as "kerogen" which decomposes upon pyrolysis or distillation to yield oil, gases, and residual carbon. It has been estimated that an equivalent of 7 trillion barrels of oil is contained in oil shale deposits in the United States with almost 60 percent located in the rich Green River oil shale deposits of Colorado, Utah, and Wyoming. The remainder is contained in the leaner Devonian-Mississippian black shale deposits which underlie most of the eastern part of the United States.
As a result of dwindling supplies of petroleum and natural gas, extensive efforts have been directed to develop retorting processes which will economically produce shale oil on a commercial basis from these vast resources.
Generally, oil shale is a fine grained sedimentary rock stratified in horizontal layers with variable richness of kerogen content. Kerogen has limited solubility in ordinary solvents and therefore, cannot be recovered by extraction. Upon heating oil shale to a sufficient temperature, the kerogen is thermally decomposed to liberate vapors, mist, and liquid droplets of shale oil, and light hydrocarbon gases, such as methane, ethane, ethene, propane and propene, as well as other products, such as hydrogen, nitrogen carbon dioxide, carbon monoxide, ammonia, steam, and hydrogen sulfide. A carbon residue typically remains on the retorted shale.
Shale oil is not a naturally-occuring product, but is formed by the pyrolysis of kerogen in the oil shale. Crude shale oil, sometimes referred to as "retort oil," is the liquid oil product recovered from the liberated effluent of an oil shale retort. Synthetic crude oil (syncrude) is the upgraded oil product resulting from the hydrogenation of crude shale oil.
Underground formations of oil shale contain various layers, deposits or strata of rich and lean oil shale. The relative richness, leanness, and depth of these layers typically vary throughout the underground formation and depend upon the particular location of the formation.
The process of pyrolyzing the kerogen in oil shale, known as retorting, to form liberated hydrocarbons, can be done in surface retorts in aboveground vessels or in in situ retorts under ground. In situ retorts require less mining and handling than surface retorts.
In in situ retorts, a flame front is continuously passed downward through a bed of rubblized oil shale to liberate shale oil, off gases, and residual water. There are two types of in situ retorts: true in situ retorts and modified in situ retorts. In true in situ retorts, all of the oil shale is retorted under ground as is, without mining or transporting any of the shale to aboveground locations. In modified in situ retorts, some of the oil shale is mined and conveyed to the surface to create a cavity or a void space in the retorting area. The remaining underground oil shale is then explosively rubblized to substantially fill the void space. The oil shale which has been conveyed to the surface is available for retorting above ground.
Typically, modified in situ retorts are formed explosively by sequentially blasting through the formation in a stepwise manner, either downwardly or upwardly. Upward blasting in conjunction with vertical crater retreat mining requires that explosives be lowered to various depths in bottomless (extremely deep) blast holes. The amount and location of the explosives are a function of the size of the retort to be formed, the relative richness and leanness of the shale to be blasted, the shape of the retort, and the desired particle size of the oil shale fragments.
Explosives can be lowered into a bottomless blast hole with a wire rope and then detonated without the use of a plug, but such technique is often inaccurate, fails to properly confine the downward thrust of the explosive forces, and causes numerous other problems. Plugs can be lowered and suspended in a blast hole with a line, but the placement of such plugs is inaccurate and unreliable for depths greater than 150 feet and is only temporary.
Prior art blast plugs, also referred to as "pigs," have been set in bottomless blast holes of oil and gas wells by laying a pipe to the desired depth in a blast hole and forcing a basket of mechanical fingers down the inside or outside of a pipe with a fluid, until the fingers come out the bottom of the pipe and expand. Thereafter, concrete is forced down the pipe with a fluid to contact and harden about the fingers.
Over the years various methods and devices for blasting in situ retorts and forming blasting bridges for oil wells have been suggested. Typifying these methods and devices are those found in U.S. Pat. Nos. 2,216,067; 3,762,771; 3,980,339; 4,043,595; 4,043,596; 4,043,597; 4,043,598; 4,146,272; 4,175,490; 4,192,553; 4,192,554; 4,201,419; 4,205,610; 4,210,366; 4,245,865; 4,262,965; 4,272,127; and Canadian Pat. No. 1,012,564. Spent oil shale has also been used to manufacture cement for a variety of purposes. Typifying the various spent shale cement manufacturing methods and uses of spent shale cement are those found in U.S. Pat. Nos. 2,592,468; 2,904,445; 3,135,618; 3,459,003; 4,120,355; 4,131,416; 4,198,097 and 4,231,617. These prior art methods and devices have met with varying degrees of success.
It is therefore desirable to provide an improved process for setting a plug in a bottomless blast hole.